1. Field
The invention relates to a process for preparing a surface on a metal workpiece for the application of a thermally sprayed layer, to a hammer or percussion brush for carrying out the process, and also to a workpiece produced according to said process.
2. Background Art
It is known that surfaces on metal workpieces intended for coating by thermal spraying have to be prepared appropriately. This can be carried out by roughening the surface. Various processes are employed for this purpose in industry, such as sand-blasting, high-pressure water jetting, brushing, milling and similar machining processes. However, these machining processes are associated with problems. For example, chips and remnants from the machining processes can remain in grooves and channels on the machined surfaces and lead to problems if they are covered and incorporated by the coating and this layer has then been honed. The depth of the grooves and channels, which have been produced by mechanical roughening, is about 100 μm. This region is flat and smooth, and therefore the thermally sprayed layer cannot readily adhere at these sites.
In cases where an engine has to be repaired by means of thermal spraying during service work, it is necessary to machine an inner zone of wear within the cylinder bore, a region having the original, smooth surface structure, which is honed for example, remaining above and underneath said zone. If a cylinder bore of this type is repaired by means of thermal spraying, the coating cannot adhere to the honed surface. Repair by means of thermal spraying is difficult particularly for engine blocks consisting of an aluminum alloy with cast-in cylinder liners owing to the aluminum lip overlapping the cylinder liner and owing to the region between the aluminum lip and the surface region on the cylinder liner to be coated. Mechanical roughening results in residual expansion stresses, and these reduce the fatigue strength of the workpiece.
A known process is also the preparation of the surface by sand-blasting with corundum particles and subsequent cleaning, before the surface coating can be applied by means of thermal spraying. In addition to the comparatively complex process step of surface cleaning, a significant disadvantage of sand-blasting with corundum particles is, in particular, that extremely small corundum particles penetrate into the surface to be coated, and can remain there despite intensive cleaning. After the surface coating has been applied, blasting particles of this type may impair the tensile adhesive strength of the coating on the previously cleaned surface considerably.
Moreover, particles of the abrasive material can also adhere to surface regions of the workpiece to be coated which are not coated and accordingly have not been blasted previously either. Abrasive material particles of this type may result in considerable problems when the workpiece is used. This can occur, for example, on the cylinder running faces of engines which have been processed in this form. Corundum particles which have remained in or on the engine components can thus result in considerable problems and, under certain circumstances, cause the engine to fail.
In order to remedy this, DE 198 40 117 A1 discloses a process for the material-removing machining of surfaces on the inner side of hollow bodies as a preparation for the application of a thermally sprayed layer, in which process some of the material which forms the inner side of the hollow bodies is removed and a surface having a defined structure and/or quality is produced. However, this known process has the disadvantage that it cannot be used to produce surface profiles having a saw-tooth effect and thus undercuts. However, since surface structures of this type provide decisive advantages with respect to the tensile adhesive strength of the thermally applied coating, this is a decisive disadvantage. In addition, the material-removing process cannot provide consistency in terms of the surface values, since the machining tools are subjected to a necessary amount of wear and thus also produce a surface structure which varies as the workpiece becomes worn. In addition, the material removal has a negative effect on the mechanical strength of the surface prepared by this process.
Furthermore, DE 27 12 863 A1 describes a percussion tool for the removal of material from surfaces, said percussion tool bearing, at one end, a bundle of metallic material-removing needles which oscillate in the longitudinal direction thereof and can thus strike in quick succession against a surface. Needle appliances of this type are usually used to remove rust or paint from surfaces. However, needle appliances of this type are also used for cleaning concrete structural parts.